Touch-sensitive controls are increasingly replacing electromechanical switches in home appliances and consumer and mobile electronics. Their popularity has gained momentum as designers have recognized that touch controls allow the creation of stylish and functional designs that differentiate products and create higher value for end users.

Quantum Research Group developed and patented its charge-transfer sensing technology in 1999. Called QT, the technology is more stable than other forms of capacitive sensing and is more tolerant of electromagnetic interference, as well as of extreme and rapid changes in temperature and humidity.

QMatrix uses a scanned, passive matrix of transverse electrodes to achieve a large number of touch keys driven by a single chip. Each sensing electrode pair contains a field drive electrode and a receive electrode. The emitting electrode is driven as a digital burst of logic pulses; the receive electrode normally collects most of the charge coupled from the emitting electrode via the overlying dielectric front panel.

This field coupling is attenuated by human touch; the human body conducts away a portion of the fields that arc through the front panel. The absorbed part of the field is reradiated by the human body back to the product via various capacitive paths.

The signal that couples through the mutual capacitance of the electrode structure is collected onto a sample capacitor that is switched by the chip synchronously with the drive pulses. A burst of pulses is used to improve the signal-to-noise ratio. The number of pulses in each burst also affects the gain of the circuit; a greater number of pulses will result in a greater amount of collected charge, and hence more signal. By modifying the burst pulse length, the gain of the circuit can be easily changed to suit various key sizes, panel materials and panel thicknesses.

After the burst completes, the charge on the sample capacitor is converted using a slope conversion resistor that is driven high, and a zero crossing is detected to result in a timer value--proportional to the X-Y electrode charge coupling--that reflects the charge absorption caused by finger touches. Because finger touches absorb charge, the measured signal decreases with touch. The burst phase causes the charge on the sample capacitor to ramp in a negative direction, and the slope conversion causes a ramp in the positive direction on the capacitor. The net effect is that the conversion process is "dual slope" and largely independent of the value of the sample capacitor. It is highly stable over time and temperature.

Moisture suppression

The matrix approach offers two moisture suppression attributes not found in any other capacitive method. First, the presence of a localized water film (such as condensation, mist or droplets) will induce only a slight increase in signal coupling. Because touch induces a decrease in signal, the contributions to signal coupling caused by moisture are in the "wrong" direction and thus do not cause false detections.

Second, the presence of larger moisture films, which can conduct away charge, are also suppressed by the use of narrow "gate times" that restrict the charge capture to a narrow time window just after the pulse edge. Because a water film can be modeled as a distributed RC network with a time-dependent characteristic, narrow gate times (on the order of a microsecond or less) heavily suppress the effects of a water film's signal potential reduction, reducing the chances of a false detection. In addition, all QMatrix devices use spread-spectrum technology that is highly effective in suppressing both radiated emissions and susceptibility to external fields.

Matrix layout and design

A matrix of many touch keys is formed using multiple X drive and Y receive lines with key electrode sets attached at intersections. Keys are scanned sequentially in time, just as an electromechanical keyboard is scanned.

Key placement is entirely arbitrary; keys can be located anywhere on a panel and need not be configured as a rectangular array. Key signals do not cross-interfere and can thus be placed immediately next to each other without creating problems. Electrodes are also immune to adjacent grounded metal and can even be placed within a millimeter of an underlying chassis or ground plane.

Although both the X and Y lines are highly resistant to the effects of ground planes and adjacent conductors, too much ground around the Y lines can absorb some of the received charge and reduce gain. This puts practical limits on Y-line capacitive loading. Key sizes, shapes and placement are almost entirely arbitrary; sizes and shapes of keys can be mixed within a single panel of keys and can vary by a factor of 20:1 in surface area. The sensitivity of each key can be set individually via commands over a serial port.

QMatrix field flow between electrodes. Touch absorbs the field, reducing the collected charge.

The electrodes are defined as two-part interleaved electrodes of almost any conductive material, such as etched pc board copper or silver or carbon ink. The electrodes are most conveniently fashioned on a conventional pc board or FPCB that is glued to the rear of the control panel. The signals are so strong and reliable that the chip, circuit and electrodes can be placed all on the same layer on the side of the board facing away from the touch surface; in this way, a very inexpensive, single-sided punched CEM-1 board can be used that is usually less than half the price of FR-4. An industrial sheet adhesive is used to bond the board to the rear of the front panel. Another inexpensive option is to use a punched PET film with screened silver on both sides to create a matrix electrode layer.

QMatrix dual-slope circuit. The pulses generate the first slope: a staircase on the sample capacitor, induced via electrode charge cross coupling. After the burst, the slope resistor is switched high to ramp the sample capacitor until a zero crossing is achieved to null its charge. The slope time required to obtain the zero crossing is the signal result. The dual-slope nature of these circuits makes them extremely stable over wide operating temperatures.

QMatrix chips contain all the signal filtering, debounce logic and automatic drift compensation that is essential to providing decades of reliable operation. Many of these devices contain failure modes and effects analysis (FMEA) self-checking routines to report any circuit failures, such as shorts and opens, to help provide fail-safe operation, making them highly prized for cooking and automotive applications.

LEDs can be placed in the middle of QMatrix keys simply by creating an opening in the board and working the drive/receive electrodes around a surface-mount LED and its pads and wiring. This can even be done on a single-sided board. Clear conductors such as Orgacon (from Agfa NV), as well as printable ITO, can be used to create clear, backlightable discrete keys when printed onto a clear substrate such as PET film. The film is laminated onto the rear of the front panel using a clear, readily obtainable adhesive sheet. A board with LEDs placed behind this layer creates the required back-illumination. Light guides and diffusers can be used to make backlighting more even.

Touchscreens

Special variants of QMatrix de- vices can drive clear capacitive touchscreens fashioned from one or two ITO layers. A singe Q- Matrix chip can drive both conventional keys and the touchscreen film in unison, creating a seamless interface to a host microcontroller for the entire front panel. The ITO film wires into the scanning matrix and provides a large number of discrete keys over an LCD display. The Whirlpool Velos oven uses this method behind a solid, unbroken sheet of curved glass to create a spectacular design.

Touchscreens made in this way are impossible to damage and are very transparent compared with resistive screens.

Hal Philipp is chief technology officer of Atmel's Capacitive Sense Division. He was previously CEO of Quantum Research Group, acquired by Atmel this year. Philipp graduated from Michigan Tech in 1975 and for part of his career worked in signal acquisition systems, notably at Tektronix, where he helped developed the architecture, control and sampling time base of the world's first commercial optical time-domain reflectometer. Philipp holds more than 30 U.S. patents and patent applications.